Not applicable.
Not applicable.
1. Field of Invention
The present invention relates to mechanical heart valve prostheses, and, in particular, to a bileaflet prosthetic heart valve with a rectangular orifice and periphery that enable full leaflet opening for improved blood flow with a single central orifice.
2. Description of Prior Art
A wide variety of heart valve prostheses have been developed to operate hemodynamically in conjunction with the pumping action of the heart to replace defective natural valves. These valves generally have annular valve bodies that function with a single occluder or a plurality of occluders that allow forward blood flow through the valve during systole and prevent retrograde flow during diastole.
The first successful mechanical heart valves were caged ball valves, pioneered by Starr and Edwards, based on the ball valve of U.S. Pat. No. 19,323 (Williams, 1858). The hemodynamic concept of the single tilting disk valve is an improvement over the caged ball valve because it reduces energy loss, and therefore it largely replaced the caged ball implant. U.S. Pat. No. 3,546,711 (Bokros, 1968) discloses a single tilting disk heart valve with journaled hinges set away from the orifice wall. U.S. Pat. No. 3,835,475 (Child, 1974) discloses a free-floating disk that is constrained by projections. U.S. Pat. No. 4,306,319 (Kaster, 1981) discloses a tilting disk heart valve with an oval, egg, or kidney shaped disk and orifice. In this valve, the disk is hinged with an axis of rotation across the largest dimension of the orifice. The tilting disk heart valves have improved flow characteristics over the caged ball valves, but still partially obstruct the central flow of blood while open.
Bileaflet valves were designed to be an improvement over the tilting disk valves; they open more smoothly, close more reliably, and have a lower profile. U.S. Pat. No. 4,078,268 (Possis, 1976) discloses a bileaflet valve with hinge axes slightly offset from the orifice diameter. U.S. Pat. No. 4,159,543 (Carpentier, 1976) discloses a bileaflet valve with diametric hinge axes in which the leaflets rotate about physical axles. U.S. Pat. No. 4,276,658 (Hanson, 1980) discloses a manifestation of a bileaflet valve where the leaflets have convex ears that fit into concave sockets for pivoting. U.S. Pat. No. 4,352,211 (Parravicini, 1981) discloses a design in which the leaflets are arcuate cylindrical shells contoured to match the round aortic duct. U.S. Pat. No. 4,451,937 (Klawitter, 1982) discloses a design with ear-guided hinges, in which the leaflets are constrained by protuberances. U.S. Pat. No. 4,655,772 (De Liotta, 1985) discloses a bileaflet design in which the leaflets are mounted on hook guides. The shift from the caged ball or tilting disk valve designs to the bileaflet designs was an improvement in safety, efficacy, and efficiency. However, bileaflet valve designs still partially obstruct the blood flow when the leaflets are open.
In an effort to decrease central occlusion, trileaflet valves have been developed. U.S. Pat. No. 4,820,299 (Phillippe, 1986) discloses a trileaflet design with hinge axes disposed from the center of the orifice at a distance that is 75 percent of the radius of the base. U.S. Pat. No. 5,207,707 (Gourley, 1993) discloses another trileaflet valve with ear-guided leaflets and conical stops. U.S. Pat. No. 5,628,791 (Bokros, 1997) discloses a trileaflet valve with another design for the hinge guidance projections. U.S. Pat. No. 5,843,183 (Bokros, 1998) discloses a trileaflet design with projection stops, as well as a significantly orifice-reducing contour to provide additional stops. U.S. Pat. No. 6,059,826 (Bokros, 2000) discloses a design with tapered leaflets to reduce cavitation in the blood.
U.S. Pat. No. 3,938,197 (Milo, 1976) discloses a valve with a pentagonal orifice mounted in a circular ring and five roughly triangular leaflets. This valve has no central occlusion, but has a significantly reduced orifice area as well as an excess of moving parts.
A major drawback of existing mechanical heart valves is the risk of thrombus formation on the valve that can foul the mechanism. Additionally, such thrombi can embolize and lead to medical conditions such as stroke, heart attack, and pulmonary embolism. Thrombosis occurs when blood is damaged by shear forces on blood corpuscles, by turbulent flow, or by chemical interactions with synthetic materials, all of which are exacerbated by cardiovascular implants. Presently, mechanical heart valve recipients receive anticoagulant drug therapy in order to avoid thrombus formation; however, this drug therapy introduces a new set of comparable health risks. Reducing the blood damage caused by the valve has the benefit of lowering the required levels of anticoagulants needed to prevent thrombosis.
Shear forces and turbulence are generated as a result of a velocity gradient in the fluid flow. In unobstructed ducted flow, the fluid velocity is a maximum in the center of the duct, and is zero at the boundary. If the occluder mechanism of a valve lies in the central region of flow when the valve is open, its surfaces induce drag on the high velocity fluid causing additional shear forces and turbulence in the fluid. With the exception of U.S. Pat. No. 3,938,197 (Milo, 1976) this is the case in all of the heart valve designs in the prior art listed above. Some valve designs move the occluders out of the direct center of flow when the valve is open by employing three or more leaflets; however, there is still significant flow occlusion. Additionally, these designs increase the number of moving parts, which, in turn, increases the probability of mechanical failure.
The use of synthetic materials such as pyrolytic carbon that have high durability and reasonably low thrombogenicity is known to the prior art. Such materials have effectively minimized material-induced thrombosis.
In accordance with the present valve, a heart valve prosthesis comprises an annular body that encloses an orifice that is generally rectangular in cross-section and leaflets that open to allow forward blood flow and close to prevent retrograde flow. The axes of rotation of the leaflet hinges are near the periphery of the orifice.
Accordingly, the primary objects of this valve are: first, to remove central flow obstructions in the valve orifice through geometric optimization; second, to obviate the need for small side orifices that split flow; third, to maximally size leaflets, limiting the required number to two; and fourth, to maintain orifice area by obviating irregular leaflet contours, taking advantage of the uniform geometry of the rectangle. These objects result in numerous advantages, detailed below, which provide a superior flow dynamic compared to the prior art. This superior flow dynamic reduces stress placed on the heart and damage to the blood, which, in turn, reduces the burden of anticoagulation therapy and the risk of thrombosis to the patient.
There are guidelines that can be used to evaluate and compare valve designs. The primary four design principles for replacement heart valves are: (1) energetic efficiency, (2) embolism prevention, (3) reduction of turbulence, and (4) reduction of blood trauma. Other principles such as noise reduction, sterilization, and material biocompatibility have largely been standardized.
These four principles deal with reducing damage to the heart (1) and blood (2-4). The major drawback of mechanical valves, vis a vis bioprosthetic valves, is that patients require anti-coagulation therapy. The dosage and frequency of this treatment attempts to minimize the competing morbidity and mortality due to stroke (too little anti-coagulant) and due to hemorrhage (too much anti-coagulant). The present valve is based on the realization that placing the leaflets in a central location has deleterious effects on all four design principles, which stem from one central cause: dividing blood flow.
Comparing energetic efficiency of valves addresses principle (1). Flow without obstructions is more efficient than flow with obstructions, and flow through a smaller orifice is less efficient than flow through a larger orifice. All surfaces provide no-slip boundaries, so two offset central leaflets have four drag surfaces, whereas two peripheral leaflets have only two drag surfaces presented to the flow. Additionally, these drag surfaces in the central case are located such that they stop the flow where it would otherwise be fastest and most efficient. In contrast, the two drag surfaces in the peripheral case are located where the flow velocity would be close to zero in the absence of leaflets, so there is little loss of efficiency.
Effective orifice area is another primary measure of valve efficiency. Due to flow mechanics, the simple act of dividing a circular viscid flow into two equal sub-flows doubles the resistance, despite the same nominal cross-sectional area. Considering the simplified geometries of a bileaflet circle, an inscribed trileaflet circle, an inscribed pentaleaflet circle, and this rectangular valve mounted in the same orifice perimeter, the normalized effective area of flow is 1.0 for the rectangular valve, 0.64 for the bileaflet, 0.56 for the trileaflet, and 0.75 for the pentagon. Each physical manifestation of these shapes has complex irregularities that affect flow efficiency, but the inherent geometric disadvantages are dominant factors.
The second result of central flow obstruction is shear damage. Clotting is the chief drawback of mechanical heart valves, in accordance with principle (2) above. One of the primary mechanisms by which valves exacerbate clotting is by inducing high shear rates. When blood shears and blood cells are ruptured, not only is the oxygen carrying capacity of blood diminished, but the damaged cells also release chemical factors that trigger coagulation. Of the shapes listed above, the rectangular orifice again proves to be most advantageous with regard to shear because its flow is most uniform. In order to pass the same flow though the effectively smaller orifices of the bi-, tri-, and pentaleaflet valves, the blood must achieve a higher peak velocity in the center of each separate sub-flow. This leads to a multiplicative effect: the peak velocity has increased, and the distance over which the blood velocity must transition from no-slip to peak had diminished. Thus shear has increased significantly, and blood is increasingly damaged.
In the final comparison of this design with the prior art in light of the above design principles, other aspects of geometry can contribute to blood trauma. The rectangular design has the shortest length of edges presented to the blood. Leaflet edges lead to turbulent energy dissipation (3) and physical crushing of blood cells between impacting surfaces (4). Additionally, the present valve does not sacrifice the number of leaflets. This valve could function with one leaflet, but the closing times and forces would increase. The prior art demonstrates that with these factors in mind, a bileaflet design is an improvement over a single-leaflet design. However, increasing the number of leaflets increases the number of parts, which increases the risk of mechanical failure. In addition, more leaflets increase the risk of performance failure, and leaflet closure complications can be catastrophic. Any damage to a leaflet that might impair simultaneous closing leads to regurgitation, which stresses the heart. Hence increasing the number of leaflets beyond two incurs additional risk that has not yet been shown to provide concomitant benefit for patients.
Taking all four of these principles into account, the rectangular valve reflects a revolutionary shift in valve design, akin to the step from caged ball to tilting disk, and from tilting disk to bileaflet.